REDOX AND ION-PAIRING THERMODYNAMICS IN SELF-ASSEMBLED MONOLAYERS

Self-assembled mixed monolayers composed of ferrocenylhexanethiol ((Fc(CH2)6SH, Fc = (eta-5-C5H5)-Fe(eta-5-C5H4)) and n-alkanethiols have been prepared on polycrystalline bulk gold electrodes. Monolayers containing ferrocenylhexanethiol and n-alkanethiols from 4 to 12 carbons long show the following trends as n-alkanethiol chain length increases: (a) the quantity of immobilized ferrocene decreases; (b) the redox potential for ferrocene oxidation shifts to more positive values. The positive shift is explained by the progressively more alkane-like environment created by the coadsorbed alkanethiols, which destablizes ferricinium relative to ferrocene. The redox potential was also found to depend on electrolyte, shifting negative with log [HC1O4]. Effective formation constants for forming ion pairs between ferricinium and several anions are calculated and compared; the values scale roughly with the energies of solvation of the anions.